1. Field of the Invention
The present invention generally relates to an electric motor driving apparatus which carries out switching of a switching device provided in a power distribution line of an electric motor with pulse width modulation (PWM) signals to control a rotational speed or the like of the electric motor. More particularly, the invention relates to an electric motor driving apparatus in which a circuit for suppressing generation of high-frequency noise (radio noise) by the switching is simplified.
2. Description of the Related Art
In an electric motor driving apparatus that drives an electric motor by PWM control, high-frequency noise (radio noise) generated by switching sometimes interferes with and influences, for example, a radio broadcast receiver mounted in a vehicle.
FIG. 3 is a diagram showing a circuit structure of a conventional electric motor driving apparatus. In the conventional electric motor driving apparatus 100, a filter circuit 104 constructed by a series circuit of a capacitor 102 and a resistor 103 is provided between a gate and a drain of a field-effect transistor (FET) 101 as a switching device, and a snubber circuit 108 constructed by a series circuit of a capacitor 106 and a resistor 107 which is in parallel with a flywheel diode 105 are disposed, to suppress the generation of high-frequency noise (radio noise). More specifically, the filter circuit 104 absorbs noise generated during operation of the FET 101 and the snubber circuit 108 protects the FET 101 from surge voltage generated during operation of an electric motor 109, thereby cooperatively suppressing the high-frequency noise (radio noise).
The electric motor 109 drives a radiator fan, a condenser fan or the like for example. Reference numeral 110 denotes a direct-current power supply such as a battery mounted on a vehicle. One of a pair of terminals of the electric motor 109 is connected to a positive pole side of the direct-current power supply 110, whereas the other of the pair of terminals of the electric motor 109 is connected to the drain of the FET 101. The flywheel diode 105 is connected in parallel with the electric motor 109. Reference numeral 111 denotes a PWM signal generator which generates and outputs PWM signals based on command signals. The PWM signals outputted from the PWM signal generator 111 are supplied to the gate of the FET 101.
The FET 101 shown in FIG. 3 is, as one example, an N-channel MOS-FET in which a diode is connected between the drain and a source and a bidirectional zener diode is connected between the gate and the source.
A resistor for detecting current 112 is connected between the source of the FET 101 and an earth terminal (ground). The PWM signal generator 111 monitors an current which flows the FET 101 (current of the electric motor) on the basis of voltage generated at both ends of the current detecting resistor 112 to carry out feedback control of duty ratio of the PWM signals so as to allow the current to be in a predetermined current, or to ensure protection against an overcurrent by stopping the output of the PWM signals when the current exceeds a previously set allowable current.
As another conventional electric motor driving apparatus, Japanese patent registration No. 3067601 discloses an apparatus in which a capacitor is provided between a gate of a switching device (FET) to which PWM signals are inputted and one of a pair of terminals of an electric motor that is disposed at an opposite side of the switching device to suppress the high-frequency noise generated at the time of on/off operation of the switching device by the capacitor.
In the conventional electric motor driving apparatus 100 shown in FIG. 3, since the filter circuit 104 and the snubber 108 as circuits for the suppression of high-frequency noise are independently arranged, respectively, there is a drawback that a total of four components or elements, namely, two capacitors 102, 106 and two resistors 103, 107, are required, rendering the structure of circuit complicated and the number of components to be large.